1. Field
The present invention relates generally to data communication, and more specifically to multi-channel communication systems (e.g., multiple-input, multiple output (MIMO) systems) with multiple transmission modes.
2. Background
In a wireless communication system, an RF modulated signal from a transmitter may reach a receiver via a number of propagation paths. The characteristics of the propagation paths typically vary over time due to a number of factors such as fading and multipath. To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used. If the propagation paths between the transmit and receive antennas are linearly independent (i.e., a transmission on one path is not formed as a linear combination of the transmissions on other paths), which is generally true to at least an extent, then the likelihood of correctly receiving a data transmission increases as the number of antennas increases. Generally, diversity increases and performance improves as the number of transmit and receive antennas increases.
A multiple-input multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, with NS≦min {NT, NR}. Each of the NS independent channels is also referred to as a spatial subchannel of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized. For example, an independent data stream may be transmitted on each of the NS spatial subchannels to increase system spectral efficiency.
The spatial subchannels of a MIMO system may experience different channel conditions (e.g., different fading and multipath effects) and may achieve different signal-to-noise ratios (SNRs) for a given amount of transmit power. Consequently, the data rates that may be supported by the spatial subchannels may be different from subchannel to subchannel, depending on the amount of transmit power allocated to the data streams and their achieved SNRs. Since the channel conditions typically vary with time, the transmission capacities of the spatial subchannels also vary with time.
A key challenge in a coded communication system is to effectively utilize the total transmit power, Ptot, available at the transmitter for data transmission on the NS spatial subchannels based on the channel conditions. Various schemes may be used to transmit data on the spatial subchannels. Each transmission scheme may require certain types of information regarding the MIMO channel and may further be premised on certain signal processing at the transmitter and receiver. In general, more complicated transmission schemes may be able to achieve spectral efficiency closer to optimum by allocating different amounts of transmit power to spatial subchannels of different capabilities and pre-conditioning the data streams prior to transmission over these subchannels. However, these transmission schemes generally require more information regarding the MIMO channel, which may be difficult to obtain at the receiver and also requires air-link resources to report to the transmitter. Less complicated transmission schemes may provide good performance over only a limited range of operating conditions, but may require less channel information.
There is therefore a need in the art for techniques to transmit data in a MIMO system to achieve high spectral efficiency and having reduced complexity.